Page 170 - Flexible Robotics in Medicine

156 Chapter 6

cannula, which allows the curved end to be exposed when the cannula is withdrawn. When
the cannula is extended, it straightens out the stylet. Therefore, by varying the length of the
stylet that is exposed, the deflection of the tip can be controlled. Using this mechanism, the
authors designed a 2-DOF (extension of the stylet and rotation of the stylet) device and can
be extended to 3-DOF. Like the aforementioned devices, this device uses a combination of
lead screws and spur gears to drive the needles, and these are mounted at the rear of the
device. This arrangement allowed for most of the device to be 20 mm in outer diameter,
which made the device convenient for handheld usage.

Contrary to the biopsy needle on which the actuation mechanism was based on, the stylet
extends, rather than the cannula retracting. This allows for the stylet to travel further, but
also needs precise motion planning to account for the curved motion path of the stylet. The
handheld device is controlled via a joystick, and the joystick motions are mapped in an
intuitive fashion to the needle’s motion. This was achieved by using encoders to read the
position of the stylet.

6.2.3 Design considerations

In summary, the fundamental design principles in designing an actuation mechanism for a
CTR can be detailed as follows:

Tubes: Tubes must be arranged from the largest diameter to the smallest diameter, starting
from the front of the robot. As a result, the inner tubes must be of a longer length compared
to the outer tubes.
Translation: Translation motor is usually offset and connected to the tube via a lead screw.

Rotation: Rotation motor can be carried on the translation axis or offset. Offset motor
would need gearing or leadscrews.

6.2.3.1 Concentric tube stiffness

One of the limiting cases in the interaction of two concentric tubes is the domination-
stiffness tube pair [24]—the outer tube’s stiffness is much higher than that of the inner tube
such that the stiffness of the outer tube can be assumed to be infinite, and thus the tube pair
will conform to the shape of the outer tube. The part of the inner tube extending beyond the
outer tube will relax to its original curvature.

Such a tube design is like that of the biopsy needle mentioned in the earlier section and is
useful for intraocular surgery. A straight shaft would allow for easy insertion into the eye,
while the flexible section would supply greater reach and dexterity compared to a
straight tool.